In the early 1930's it was proposed that the quality of a fuel be based on a comparison of its ignition delay time in a diesel engine to that of a blend of two reference fuels. The resulting development was the "cetane scale" in which a fuel was assigned a "cetane number.". The reference fuels used were cetane, C.sub.16 H.sub.32, and mesitylene, C.sub.9 H.sub.12, with cetane numbers of 100 and 0, respectively. These particular fuels were chosen because cetane burned readily in a normal engine, while mesitylene did not burn at all. In 1935 the ASTM adopted this form of diesel rating system using cetane, C.sub.16 H.sub.34, and alphamethylnaphthalene, C.sub.11 H.sub.10, as the reference fuels. The former was assigned a cetane number of 100, while the latter was given a cetane number of 0. In 1962 the ASTM added heptamethylnonane, C.sub.16 H.sub.34, to the cetane sprocket as an intermediate, low-ignition-quality fuel with a cetane number of 15.
The currently used cetane rating procedure has a number of shortcomings. One of the basic problems with the cetane procedure is that neither the engine nor the test conditions are representative of current engine design or typical operating condition.
A major criticism of the CFR engine test for measuring the cetane number of a fuel sample has been the poor repeatability and reproducibility of the results. The current procedure involves the use of a Waukesha CFR engine equipped with a cylindrical prechamber. The prechamber has a movable end plate which is used to change the volume of the prechamber and thus the compression ratio. The specified operating conditions of the test are equivalent to a high-speed idle test, with the speed set at 900 rpm and fuel flow set at 13 ml/min (equivalent to an air-fuel ratio of approximately 30).
In summary, the basic problems associated with the current ASTM cetane rating procedure are that neither the engine nor the test conditions are representative of modern practice. In addition to this basic problem, there are several more practical problems with the current system. These can be summarized as follows: (1) Cetane number does not correlate directly with engine startability in modern engine designs; (2) The standard deviation of the measurement is high (0.7 CN) in a given laboratory, and laboratory-to-laboratory variations can be as high as 4 to 6 CN; (3) The cetane number does not provided an accurate measure of ignition quality of fuels in which the ignition delay dependence on temperature and type of ignition (single-stage or two-stage) differ from the reference fuels; i.e., the procedure is realistically limited to petroleum diesel fuels; (4) The time and cost factors associated with the current procedure (ASTMD 613) are prohibitive.
Clearly a new approach to rating ignition quality should address the underlying problems of adequate representation of modern practice, in addition to the other problems listed above. If an engine-based technique is employed, the rating techniques must be performed in an engine configuration and at test conditions representative of current practice. Ideally the test engine should incorporate direct-injection and be capable of speeds up to 2500 rpm to be representative of the current design of heavy-duty engines. Features which are deemed to be important in the performance of the evaluation are variable compression ratio, accurate control of inlet air and coolant temperatures, and accurate control of fuel flow rates. An engine capable of meeting the above discussed criteria has heretofore been lacking in the prior art.